Pyrans tetrahydro-, cleavage

Bromotrimethylsilane has proven to be useful for a wide variety of applications most of them being reviewed. Other recent applications are mild cleavage of oxiranes, the synthesis of glycosyl bromides, the selective cleavage of tetrahydro-2,5-dimethoxyfuran and tetrahydro-2,6-dimethoxy-pyran, the cleavage of esters and ethers, and the synthesis of benzyl bromides. ... 

Lewis acids have also been found to promote the electrophilic cleavage of cyclobutanes. The key feature of this reaction can be depicted by the conversion of the chiral 4-(2-hydroxy-6-methoxyphenyl)-6,6-dimethylbicyclo[3.1.1]heptan-2-ones 5 and 4-(2-methoxy-6-hydroxyphenyl)-6,6-dimethylbicyclo[3.1.1]hept-3-en-2-ones 7 to (6a5, 10a/ )-(-)-l-methoxy-6,6a,7,8,10,10a-hexahydro-6,6-dimethyl-9//-dibenzo[b,c/]pyran-9-ones 6 and (6aS)-( + )-methoxy-6,6a,7,8-tetrahydro-6,6-dimethyl-9//-dibcnzo[b//]pyran-9-ones 8, respectively, on treatment with either tin(IV) chloride in chloroform or with aluminum trichloride in dichloromethane.52... 

Zhou et al. [18] reported the use of calcium in ethylenediamine for reductive cleavage of dihydropyrans (Scheme 4.6). In 5,6-dihydro-2H-pyrans 19, the allylic C-O bond is cleaved selectively to give a mixture of alkenyl alcohol 20 and saturated alcohol 21 in a 98 2 ratio with an overall yield of 58-60%. Application of the same reducing agent to 3,4-dihydro-2H-pyran (22), however, led to tetrahydro-pyranyl pentyl ether 23 in 51% yield. This involves reduction followed by an addition process. 

Another valuable, synthetic intermediate for the preparation of 2-substituted derivatives 141 and 142 is 1,6 2,3-dianhydro-4-0-benzyl-/3-D-mannopyranose (116), where, for 141 and 142, X is H,155,376-632 D,153 MeO,135 OH,52 PhCH20,442,629 NH2,633,678 NH-alkyl,737 N3,337 F,380,306 or I.384 Similar cleavages were described for l,6 3,4-dian-hydro-2-0-benzyl-/3-D-galactopyranose,158,442,5320,534,542,629,740,758,759 and for the corresponding 2-O-methyl,264 2-O-allyl,434 2-0-(tetrahydro-pyran-2-yl),496 2-deoxy-2-fluoro,380,381,507 and 2-azido-2-deoxy deriva-... 

---